logging-winston: make winston a peerDependency (#2453)
logging-winston: make winston a peerDependency (#2453)

Users of this library need to depend on winston directly. There are
scenarios where the user, or other libraries may depend on a semver
incompatible version of winston. Not only does it end up installing an
unnecessary copy of winston; it may also lead to our transport not
being visible in `winston.transports` that the user has visibility to.
callmehiphop/gcloud-node,callmehiphop/gcloud-node,GoogleCloudPlatform/gcloud-node,jgeewax/gcloud-node,GoogleCloudPlatform/gcloud-node,jgeewax/gcloud-node,googleapis/google-cloud-node,googleapis/google-cloud-node,googleapis/google-cloud-node,GoogleCloudPlatform/gcloud-node,googleapis/google-cloud-node